Modbus MCP NPX Server

Overview

• NPX-friendly Model Context Protocol server exposing Modbus tools over stdio.
• Supports Modbus TCP, UDP, and Serial (RTU) via modbus-serial.
• Mirrors the Python Modbus MCP: core read/write, typed decode/encode, tag map, retries, chunking, and health.

Install/Run

• Run via npx (when published) or clone and build locally.

Local build

• From this folder:
  • npm install
  • npm run build
  • node build/index.js

Claude Desktop config (example) { "mcpServers": { "Modbus MCP (NPX)": { "command": "modbus-mcp", "env": { "MODBUS_TYPE": "tcp", "MODBUS_HOST": "127.0.0.1", "MODBUS_PORT": "502", "MODBUS_DEFAULT_SLAVE_ID": "1" } } } }

Environment

• MODBUS_TYPE: tcp | udp | serial (default: tcp)
• MODBUS_HOST: host for TCP/UDP (default: 127.0.0.1)
• MODBUS_PORT: port for TCP/UDP (default: 502)
• MODBUS_DEFAULT_SLAVE_ID: default unit id (default: 1)
• MODBUS_SERIAL_PORT: serial device path (default: /dev/ttyUSB0)
• MODBUS_BAUDRATE: serial baud (default: 9600)
• MODBUS_PARITY: N | E | O (default: N)
• MODBUS_STOPBITS: stop bits (default: 1)
• MODBUS_BYTESIZE: data bits (default: 8)
• MODBUS_TIMEOUT: seconds per request (default: 1)
• MODBUS_MAX_RETRIES: retry attempts (default: 2)
• MODBUS_RETRY_BACKOFF_BASE: backoff seconds (default: 0.2)
• MODBUS_TOOL_TIMEOUT: per-tool timeout (seconds, optional)
• MODBUS_WRITES_ENABLED: true/false (default: true)
• REGISTER_MAP_FILE: path to tag-map JSON (optional)

Tools

• read_register: address, slave_id?
• write_register: address, value, slave_id?
• read_coils: address, count, slave_id?
• write_coil: address, value, slave_id?
• read_input_registers: address, count, slave_id?
• read_multiple_holding_registers: address, count, slave_id?
• read_discrete_inputs: address, count, slave_id?
• write_registers: address, values[], slave_id?
• write_coils_bulk: address, values[], slave_id?
• mask_write_register: address, and_mask, or_mask, slave_id?
• read_device_information: slave_id?, read_code?, object_id?
• read_holding_typed: address, dtype, count?, byteorder?, wordorder?, scale?, offset?, slave_id?
• read_input_typed: address, dtype, count?, byteorder?, wordorder?, scale?, offset?, slave_id?
• list_tags
• read_tag: name
• write_tag: name, value
• ping

Result shape

• Every tool returns JSON content: { success, data, error, meta }.

Tag map (optional)

• Example file referenced by REGISTER_MAP_FILE: { "PumpSpeed": { "table": "holding", "address": 100, "dtype": "float32", "count": 1, "byteorder": "big", "wordorder": "big", "slave_id": 1 }, "ValveOpen": { "table": "coil", "address": 5, "slave_id": 1 }, "Alarm": { "table": "discrete", "address": 10, "count": 4, "slave_id": 1 } }

Notes

• Uses modbus-serial which supports TCP, UDP, and Serial (RTU). Function coverage: FC1, FC2, FC3, FC4, FC5, FC6, FC15, FC16, FC22, and MEI 43/14.
• Reads are chunked to protocol limits: coils/discretes=2000, registers=125.
• Typed decode/encode supports int16/uint16/int32/uint32/float32/int64/uint64/float64 with byteorder and wordorder semantics.